A skylight or glass roof such as is used in a greenhouse is typically provided with a shade that can be drawn to keep out light when desired. Such shades can be mounted externally or internally, the former normally being preferred when inside space is at a premium.
The standard external-mount shade assembly comprises a rod mounted at the upper edge of the window and around which the shade is wound. Mechanism extending down from the leading edge of the shade can draw it down to the lower eave edge.
Thus with this system the assembly always blocks at least part of the roof window. Thus some of the benefit of the window is lost. In addition the shade assembly creates an unattractive encumbrance on the roof plane and can catch debris and snow.